Tertiary Treatment

Slowing Seawater Intrusion

Salinas Valley is known as the Salad Bowl of the Nation for the wide variety of row crops cultivated in the rich valley soil. As agriculture is the backbone of the region’s economy, water is the life blood.  Recycling water for irrigation in Northern Salinas valley reduces aquifer pumping, thereby retarding the advancement of seawater intrusion (see Our Histoy page).

To our knowledge, Monterey One Water operates the world’s largest water recycling facility designed to irrigate freshly edible food crops.


It became evident during the early 1970s that the quality of northern Monterey County's groundwater supply was deteriorating because of extensive withdrawal of groundwater for agriculture. This overdraft led to an increasing problem of seawater intrusion, which was threatening the multibillion dollar agricultural industry and the drinking water supply for the City of Salinas.

In the mid-1970s, a group of community leaders began discussing the idea of recycling wastewater. The objective was to retard the advance of seawater intrusion by supplying supplemental irrigation water to farmland in the northern Salinas Valley. This would significantly reduce the draw of water from the underground aquifers.

Using highly treated wastewater to irrigate landscaping has been practiced for years. However, using recycled water for food crops was relatively new. This lead to an extensive 11-year Monterey Wastewater Reclamation Study for Agriculture (MWRSA; PDF) which begun in 1976. It consisted of full-scale field tests using reclaimed wastewater on various food crops, including artichokes, celery, broccoli, lettuce, and cauliflower. The final results of this research proved that recycled water is safe for the irrigation of food crops that are consumed without cooking. In addition, reclaimed water produced higher crop yields with better quality and appearance than crops grown with well water. The conclusions of the study were published in 1987. Today, this definitive report is used as the standard in countries all over the world.

Monterey County Reclamation Projects

In 1992, MRWPCA (now M1W) and the Monterey County Water Resources Agency formed a partnership to build the Monterey County Reclamation Projects:  The Salinas Valley Reclamation Project (SVRP) recycled water plant and the Castroville Seawater Intrusion (CSIP) distribution system. As long-time project partners, M1W is contracted with the Monterey County Water Resources Agency (MCWRA) to operate and maintain CSIP.

Since 1998, the Monterey County Reclamation Projects have proven to be successful as a safe and reliable water supply for the 21st Century. In addition to retarding seawater intrusion and protecting our drinking water supplies, water recycling also reduces wastewater discharge into the Monterey Bay National Marine Sanctuary.

Salinas Valley Reclamation Project (SVRP)

At work on the water recycling pipelineThe $75 million reclamation projects were completed in 1997 after three years of construction with low interest State Revolving Loan Program funds. These project construction costs are paid by Salinas Valley property assessments and water use fees approved by a MCWRA Proposition 218 election and enacted on October 6, 1992 by the Monterey County Board of Supervisors by Ordinance 3635 and Ordinance 3636.

The SVRP came online September of 1997 and began distributing irrigation water through CSIP on April 15, 1998. During the fall of 1997 and early 1998, food safety tests were conducted on the SVRP water to determine removal of new “emerging pathogens” that were not evaluated in the earlier Monterey Wastewater Reclamation Study for Agriculture (PDF). The new results again showed no viable microbes of public health concern:

Additional Reading:

Efficacy of Pathogen Removal (PDF) During Full-scale Operation of Water Reuse Facilities in Monterey California (presented at the IWA 4th International Symposium on Wastewater Reclamation and Reuse November 12-14, 2003, Mexico, City)

Recycled Water Food Safety Study (PDF) for Monterey County Water Recycling Projects. Prepared by the Water Quality and Operations Committee. Sponsored by Monterey County Water Resources Agency and Monterey Regional Water Pollution Control Agency. August 1998.

SVRP Tertiary TreatmentSalinas Valley Reclamation Project Facility

The Salinas Valley Reclamation Project (SVRP) treats wastewater to advanced tertiary level. The resultant recycled water meets all State Standards for recreational uses, including unrestricted use on freshly edible food crops. The facility is sized to produce a maximum of 29.6 million gallons of recycled water per day. This is the equivalent of one foot of water over 91 acres of land. The SVRP is the largest sewage treatment installation in the world to recycle wastewater for freshly edible food crops. During the summer growing season, M1W recycles 100 percent of its intake water!

Tertiary Treatment — A 3-Step Process

Tertiary, or recycled water treatment, moves secondary treated wastewater through the Salinas Valley Reclamation Project (SVRP) water recycling facility where it undergoes a 3-step chemical and filtration processes.  This is the same process used to treat drinking water.

Tertiary Filter Model Tertiary Treatment Step 1In the first step occurs in a Flocculation Basin where an alum polymer is introduced to secondary treated water to bind together any remaining dissolved organic matter.  This creates tiny clumps called flocs.

Tertiary Treatment Step 2Water quality monitoringIn the second step, the flocs are then removed in the Tertiary Filters.  Treated water filters through a 6-foot bed of anthracite coal, sand and gravel in which the flocs are trapped.

Tertiary Treatment Step 3After filtration, the water flows to the third step for disinfection in the Chlorine Contact Basins.   Disinfection destroys bacteria and germs by maintaining a specific chlorine level in the water for two hours. The final product is clear, odorless and safe to use for irrigation of food crops.  It also meets public contact standards for swimming pools.

The water is continuously monitored so that safety standards are consistently maintained.  Laboratory analysts also perform frequent water quality and agronomic suitability tests.  Each month, M1W performs hundreds of analyses more than are required.  These additional measures ensure water quality and safety.

The SVRP pond can hold one days supply of water, up to 80 Acre-Feet.After treatment, the recycled water is held temporarily in an 80-acre-foot Storage Pond before it is distributed to farmlands via the underground pipeline system, the Castroville Seawater Intrusion Project (CSIP). The purple color pipe is an international color for recycled water. You can see them throughout the 12,000-acre CSIP pipeline distribution system.

Castroville Seawater Intrusion Project (CSIP)

The recycled water is distributed to 12,000 acres of farmland in Northern Monterey County through MCWRA’s Castroville Seawater Intrusion Project (CSIP) which includes 45 miles of pipeline and 22 supplemental wells. To learn more about the recycled water projects, visit the Slowing Seawater Intrusion page.

CSIP Area ServedDuring the rainy season, when the growers don't need irrigation water, secondary treated water is safely discharged two miles into the Monterey Bay through M1W’s outfall pipe. However, an average of 60% of M1W’s water is recycled each year. In the future, M1W hopes to utilize 100% of this precious resource for our communities.

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